Within a relational database system, data is stored in various types of data containers. Such data containers typically have a structure. The structure of a container is imposed on the data it contains. For example, tables are organized into rows and columns. When data is stored in a table, individual data items within the data are stored in the specific rows and columns, thus imposing a structure on the data.
Typically, the structure imposed on the data corresponds to logical relationships within the data. For example, all values stored within a given row of a table will typically have some logical relationship to each other. For example, all values within a given row of an employee table may correspond to the same employee.
Outside of database systems, the degree to which electronic data is structured may vary widely based on the nature of the data. For example, data stored in spreadsheets is generally highly structured, while data representing visual images is generally highly unstructured.
XML (eXtensible Markup Language) is becoming increasingly popular as the format for describing and storing all forms of data. Thus, providing support for storing, searching and manipulating XML documents is an extremely important problem for data management systems today.
Information about the structure of specific types of XML documents may be specified in documents referred to as “XML schemas”. For example, the XML schema for a particular type of XML document may specify the names for the data items contained in that particular type of XML document, the hierarchical relationship between the data items contained in that type of XML document, datatypes of the data items contained in that particular type of XML document, etc.
Unfortunately, although XML documents are structured, the structure of XML documents is largely ignored by database systems when database systems are used to store XML documents. For example, a highly structured XML document, containing multiple values for multiple attributes, may simply be stored as if it were an atomic undifferentiated piece of data in a single CLOB column of a table. When XML documents are stored in this fashion, the performance and scalability features of the database cannot be fully exploited to access the XML data.